Enhancement of the corrosion resistance and mechanical properties of nanocrystalline titanium by low-temperature annealing
Introduction
Commercially pure titanium displays high corrosion resistance in simulated body fluids and good biocompatibility [[1], [2], [3]]. Nevertheless, its low mechanical strength and wear resistance significantly reduce its usefulness in implantology. The addition of alloying elements such as aluminium and vanadium significantly improves the mechanical properties of titanium. Currently, Ti-6Al-4 V is the most widely used titanium biomedical alloy. Despite its satisfactory corrosion resistance, elevated concentrations of metal ions have been detected in the human body after prolonged contact with implants made from this alloy [4]. Investigations have indicated that the presence of vanadium and aluminium ions could have serious adverse effects on human health [5]. Vanadium ions are potentially cytotoxic [6], whereas aluminium is expected to magnify the risk of metabolic bone disease (e.g. osteomalacia) and neurological disease (e.g. Alzheimer’s) [7]. The simultaneous action of wear and corrosion (tribocorrosion) aggravates the release of metal ions. It has been observed that at interfaces such as dental implant/bone, the release of aluminium and vanadium ions is enhanced due to fretting (tribocorrosion involving micromotions). Thus, extensive work is underway to investigate a new group of aluminium- and vanadium-free titanium alloys [[8], [9], [10], [11]]. An alternative method for improving the mechanical properties of commercially pure titanium, without adding undesirable alloying elements, is grain refinement. It has been reported that nanostructure leads not only to a substantial increase in mechanical strength but also to a slight reduction in elastic modulus [12]. This lower stiffness results in less stress-shielding and reduces the risk of bone resorption around the implant [5].
In recent years, substantial progress has been made in nanocrystalline and ultrafine-grained materials processed using SPD (severe plastic deformation) methods [[13], [14], [15], [16], [17], [18], [19], [20]]. The development of SPD technologies has made it possible to produce nanotitanium-based dental implants. It has been observed that the healing process is faster in the case of nanocrystalline titanium [5], which reduces the risk of a rapid loss of implant stability (known as stability dip) occurring a few weeks after insertion [[21], [22]]. This may be an advantage when immediate implant loading is considered.
There are numerous studies in the literature showing that nanostructure also has a beneficial influence on titanium corrosion resistance [2,23,24]. This phenomenon is mainly due to the high volume fraction of grain boundaries. Owing to their higher energy, grain boundaries are preferential sites for the nucleation of protective layers. A high number of grain boundaries could contribute to a more rapid formation of a passive layer on nanocrystalline titanium than on microcrystalline titanium [2,23,24]. The latest data show opportunities for further improving the properties of nanocrystalline titanium. Low-temperature annealing (within the range of nanotitanium thermal stability) seems highly promising. The annealing of deformed metals usually results in a decrease in mechanical strength and increased ductility [25]. The elevated energy state of nanocrystalline metals produced by SPD techniques is a driving force of grain growth, which results in a reduction in their mechanical strength at lower temperatures compared to their microcrystalline counterparts [26]. However, it has been observed that annealing at low temperatures, which does not lead to significant growth of nano-grains, may cause an unusual hardening of nanocrystalline titanium. This phenomenon is opposite to that of conventional coarse-grained materials, where annealing usually lowers strength [27]. It has been reported that low-temperature annealing leads not only to an improvement in the hardness of nanotitanium, but also to an increase in its ductility [28]. High strength and ductility are key mechanical properties, so an opportunity to improve both simultaneously by means of this fast, simple and inexpensive process is very promising. Hardness enhancement was observed not only for nanotitanium but also for other metals and alloys thereof obtained using SPD methods [[29], [30], [31]]. The literature contains a few hypotheses explaining this phenomenon. One of them assumes that the increase in hardness is due to a significant reduction in mobile dislocations and to changes in the character of grain boundaries [14,25]. These factors also affect the corrosion behaviour of nanomaterials [32]. The effect of low-temperature annealing on nanocrystalline titanium's corrosion resistance has not been studied sufficiently. There have only been a small number of investigations, and these present contrary opinions about this phenomenon [[33], [34], [35]].
The aim of this work is to investigate the effect of low-temperature annealing on the corrosion resistance and mechanical properties of nanocrystalline titanium fabricated by means of hydrostatic extrusion. Moreover, the study provides an insight as to how changes in these properties relate to the rearrangement of structural defects triggered by heat treatment.
Section snippets
Materials and methods
The initial material was commercially pure titanium (Ti Grade 2) in the form of a 50-mm diameter rod. In order to obtain a nanostructure, the material was processed in multiple passes of hydrostatic extrusion (HE). The hydrostatic extrusion was executed at room temperature with a graphite lubricant. After 7 passes of HE, the diameter of the titanium rod was reduced from 50 to 7 mm, which corresponded to a true strain ε = 3.9. Deformation process was carried out at the Institute of High Pressure
Microstructure and mechanical properties
The microstructure of the coarse-grained titanium contained equiaxed grains (with a mean diameter of 29 μm) with a high number of twin boundaries (Fig. 1). Multiple hydrostatic extrusion with a true strain of 3.9 led to a refinement of the titanium grains to nanometric size (Fig. 2a). Titanium nanostructure after HE consisted of highly-deformed grains with a non-uniform contrast relating to high internal stresses (Fig. 2a). It was difficult to assess the grain size because the grain boundaries
Discussion
Annealing of deformed metals lead to rearrangement of dislocations and relieving stresses which in case of conventional microcrystalline materials improve ductility and reduces strength. However, low-temperature heat treatment was able to yield the opposite effect for nanomaterials [25]. In this study, it was shown that low-temperature heat treatment can beneficially affect not only the hardness of nanocrystalline titanium, but also its corrosion resistance. The shorter annealing time made it
Conclusions
In this study, the possibility of further improving the properties of nanocrystalline titanium by low-temperature annealing was investigated. Moreover, this paper proposes an explanation of the phenomena observed based on the changes induced in the nanostructure by low-temperature annealing. The main findings of this work can be summarized as follows:
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A short time annealing at 250 °C enhances both the hardness and corrosion resistance of nanocrystalline titanium.
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Annealing for 30 min led to an
Acknowledgments
This research was funded by The Faculty of Materials Science and Engineering, Warsaw University of Technology.
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